Ink jet printers, and in particular, drop-on-demand ink jet printers having print heads with acoustic drivers for ink drop formation are well known in the art. The principle behind an impulse ink jet of this type is the generation of the pressure wave in an ink chamber and subsequent emission of ink droplets from the ink chamber through a nozzle orifice, the orifice extending through an ink jet print head nozzle orifice surface. A wide variety of acoustic drivers have been employed in ink jet print heads of this type. For example, the drivers may consist of a transducer formed by piezoceramic material bonded to a thin diaphragm. In response to an applied voltage, the diaphragm displaces ink in the ink chamber and causes a pressure wave and a flow of ink to one or more nozzles resulting in the jetting of a drop of ink toward print media. Other types of acoustic drivers for generating pressure waves in ink in response to drive signals include heater-bubble source drivers (so called bubble jets) and electromagnet solenoid drivers.
Known ink jets typically combine an ink jet print head with a reservoir for supplying ink, including plural colors of ink, to various nozzles of the ink jet print head. Also, it is common to shuttle or scan an ink jet print head transversely across print media as the print media is being printed by the ink jet print head.
During printing, drops of ink tend to collect in and around the ink jet nozzle orifices. When the ink does build up, it can prevent drop ejection or cause improper ink drop trajectory and nonuniformity in ink drop size. In one approach to minimizing problems associated with ink build up on a nozzle orifice surface, the surface adjacent to nozzle orifices is coated with an anti-wetting material. In one example, Teflon.RTM., by Dupont Corporation, is used as the anti-wetting material and is deposited on the surface of the nozzle orifice plate as described in co-pending patent application Ser. No. 215,126 and entitled, "Modified Ink Jet Printing Head Method for Producing Ink Jet Printed Images". Even with anti-wetting coatings, the nozzle surface typically must be cleansed of excess ink periodically in order to maintain consistently clean orifices during printing.
In addition to ink build up, contaminants and air bubbles from ink delivered to the nozzle orifices can plug or partially obstruct these orifices, resulting in unsatisfactory printing performance by the printer. Also, air bubbles can be drawn into the nozzle orifices causing their failure. This is a problem in both aqueous inkjet printers and ink jet printers using phase-change or hot-melt ink of the type in which ink is heated and ejected in liquid form from the nozzle orifices and then solidifies on the print media. When hot-melt ink cools in an ink jet print head when the printer is off, the ink tends to contract. Contraction of ink under these conditions can draw air bubbles into the nozzle orifices. Unless eliminated, these air bubbles, as mentioned above, can interfere with or prevent subsequent printing operations. Also, lint, dust and fibers from the environment, including from print media, can travel to the nozzle orifice surface and interfere with ink drop formation, unless cleaned.
A number of prior art approaches for cleaning and maintaining ink jet print heads have previously been disclosed. For example, air bubbles have been purged from an ink reservoir chamber and nozzle orifice plates by accelerating the flow of ink through such components during a purging operation. Such purging is disclosed in U.S. Pat. No. 4,727,378 to Le, et al. and in a service manual for a colorgraphics copier, having Model No. 4692, made by Tektronix, Inc. of Beaverton, Oreg.
U.S Pat. No. 4,800,403 to Accattino, et al. sets forth another approach for cleaning ink jet printing nozzles. As recited in this patent, when an the ink jet print head is moved to a parked position, a container and cover cup is shifted by a complex drive mechanism against a nozzle orifice surface of an ink jet print head. A suction pump is then activated to apply a vacuum to the container and draw ink from the ink jet print head. Simultaneously with the application of the suction, the printing elements are operated to expel ink from the ink jet print head. Following the suction cycle, a valve is opened to vent the interior of the container to atmosphere and permit the removal of the container from the print head. At the end of the purging operation, a resilient disk is rotated to bring a wiping lobe in position to engage the nozzle orifice surface as the print head is moved past the disk.
The complex drive mechanism (as best seen in FIGS. 1 and 2 of this patent) would be relatively expensive and subject to mechanical failure. In addition, this reference is understood to operate by sucking ink through the pump utilized to apply the vacuum. Although this is satisfactory for aqueous inks, phase-change inks would tend to solidify in the tubing and pump, rendering this system inappropriate for such applications.
International Patent Application to Howtek, Inc. (published May 18, 2989 as WO 89/04255) discloses an ink cleaning system in which a seal selectively closes an ink reservoir. Pressurized air is then supplied to the reservoir to force the discharge of ink from nozzle orifices for cleaning purposes. As best seen in FIG. 3, ink is discharged into a gutter assembly, which delivers the ink to a disposable waste container. This system is disclosed as being suitable for cleaning hot-melt ink jet printers. Among the drawbacks of this system is that it does not address problems associated with cleaning a nozzle orifice surface and the degradation in print quality resulting from contamination of the surface surrounding nozzle orifices.
U.S. Pat. No. 4,829,318 to Racicot, et al. discloses a system for purging and cleaning an ink jet print head. During a purging operation, a vent to an ink reservoir is plugged and the reservoir is pressurized to eject ink from nozzle orifices. A ribbon of ink absorbing cloth or fabric is wiped across the surface of a nozzle orifice plate to clean ink from the plate. This patent mentions wiping across the printing face at a rate of 0.13 inch per second and also mentions that the wiping action is initiated at the same time the purge pump is activated. Even though the wiping cloth in this patent is described as lint free, small particles of dust can be transferred from the wiping cloth to the nozzle orifice surface, causing problems in printing.
Devices such as described in the Racicot, et al. patent and the aforesaid Howtek, Inc. International Application suffer from problems in coupling a pressure source to a vent opening. A specialized gasket arrangement is used in the Racicot, et al. patent to address this drawback. In addition, by increasing the pressure of gas, for example air, over ink in an ink reservoir, additional gas may be driven into the ink, which can result in bubbles forming in the ink jet print head after the pressure is relieved.
U.S. Pat. No. 4,577,203 to Kawamura discloses an ink jet recording apparatus in which a suction chamber positioned against a nozzle orifice surface for drawing ink and air out of the ejection nozzles. In addition, a motorized belt having ink scraping projections is operated to scrape dust and other impurities off the nozzle orifice surface. This scraping mechanism and associated motor is mounted to the suction chamber for movement toward and away from the nozzle orifice surface with the movement of the suction chamber. A solenoid is used to move the suction chamber and cleaner. U.S. Pat. No. 4,745,414 to Okamura, et al. is similar to the Kawamura reference in that it has a suction cap and cleaning blade driven by a worm wheel drive. Both of these references are understood (see FIG. 2 of the Kawamura patent and FIG. 4 of the Okamura, et al. patent) to deliver ink sucked from a nozzle orifice surface by way of tubing to a pump. In the case of hot-melt ink, ink can solidify in such cleaning elements and cause them to fail. These devices also suffer from other drawbacks.
U.S. Pat. No. 4,970,535 to Oswald, et al. describes an ink jet print head face cleaner in which air is directed substantially across the entire nozzle surface of an ink jet print head for cleaning purposes. This air is passed through a chamber which is sealed against a nozzle orifice plate during the air cleaning operation. When this chamber is removed, ink remaining on the orifice surface would not be satisfactorily cleaned by the air. Also, by directing an air stream across substantially an entire surface of a nozzle orifice plate, the cleaning effect of the air stream is diluted, as opposed to being concentrated for more effective cleaning.
Therefore, a need exists for an improved ink jet print head maintenance system directed toward overcoming these and other disadvantages of the prior art.